Yeast in sourdough breads

Linda Collister, in her book, "bread, from sourdough to rye", speaks glowingly about "never having to buy yeast again" but the recipes she then gives for sourdough bread have commercial yeast in them, albeit very small portion, but commercial nonetheless. I've looked up other recipes for sourdough bread, and most of them have small amounts of commercial yeast in them. Is it not possible to make bread relying on the starter alone to raise it?